1. Field of the Invention
The present invention relates to a coupling for high pressure fluid.
2. Description of the Related Art
In a common rail type Diesel engine, for example, fuel under a high pressure of more than 100 MPa, discharged from the fuel feed pump, is fed into the main pipe via a connecting pipe connected to the main pipe, and the fuel under high pressure fed into the main pipe is distributed to the fuel injectors via corresponding connecting pipes connected to the main pipe. In this case, generally speaking, the connecting pipes cannot be directly welded to the main pipe, and thus each connecting pipe is normally connected to the main pipe via a coupling.
In a known coupling for connecting a connecting pipe to the main pipe, the coupling member is fitted to the main pipe, and then welded thereon, and a groove, connected to the high pressure fuel flow passage in the main pipe and diverging toward the outside of the main pipe, is formed on the outer face of the main pipe. The tip portion of the connecting pipe is pressed against the groove by urging the connecting pipe onto the main pipe by tightening a nut fitted to the coupling member (see Japanese Unexamined Utility Model publication Nos. 64-4994 and 64-4995).
In general, when assembling the connecting pipes, when one end of each of the connecting pipes is connected to the fuel pump or the fuel injector, the other end of each connecting pipe is not normally aligned with the axis of the groove formed on the main pipe, i.e., the other end of the connecting pipe is normally inclined relative to the axis of the groove. Nevertheless, where the connecting pipe has a high flexibility, even if the other end of the connecting pipe, i.e., the tip portion of the connecting pipe, is not completely aligned with the axis of the groove, as mentioned above, and the coupling member is welded to the main pipe, the connecting pipe can be deformed so that the tip portion of the connecting pipe is aligned with the axis of the groove when the tip portion of the connecting pipe is urged toward the main pipe by the nut. Accordingly, in this case, no serious problems arise.
Where, however, a fuel under high pressure is to flow within the connecting pipe, it is necessary to considerably increase the thickness of the wall of the connecting pipe, and as a result, the flexibility of the connecting pipe becomes extremely low. Accordingly, in this case, if the tip portion of the connecting pipe is forcibly aligned with the axis of the groove, by urging the tip portion of the connecting pipe toward the main pipe by the nut, an excessive stress is generated in the connecting pipe and in the portion of the coupling member welded to the main pipe, and as a result, a problem arises in that the connecting pipe and the welded portion of the coupling member are damaged.